Andrew Buchan: 'I have a well of darkness in me'

'I love going unnoticed," says Andrew Buchan. But sometimes that doesn't happen. One afternoon, for instance, the actor was in the gift shop at Liverpool football club, happily examining keyrings. A man came up to him and said: "You're that lad off the telly." He replied: "Very possibly, yeah." And the man said: " I've got a story for you."

As we sit in the genteel cafe of a swanky London hotel, with a playerless piano tinkling Lloyd Webber tunes in the background, Buchan acts out a little two-hander, playing both thickly accented scouser and Andrew Buchan, baffled off-duty thesp. "So the man says, 'Last summer I was housesitting for me girlfriend Carole, who was off to Cyprus with her mate.' And I'm like, 'Right, I see.' Then he said, 'I was looking after her two cats, Jeremy and Kyle.'"

Oh come on, I laugh, you're making this up. "Hand on heart," says Buchan. Back to the story. "'Around midnight it was, and her loopy juice ex-boyfriend comes round with his mate and pushes the front door down, and then comes at me with a bat. I was there minding me own, watching telly.' 'Right, I see.' 'Unluckily for this prick, I'm quite handy myself, so I knocked the little one down fast and took this prick out into the garden in a headlock and beat him to a pulp. Police and ambulance came and turns out he dies.' And I'm like, 'Ri-ight.' 'Yeah, but as it goes, he was on the top 10 wanted list, so I got an open ticket from the coppers and they said you've done us a favour. Now, back in Newton-le-Willows, where I'm from, everybody calls me The Fixer.'"

The Fixer is the name of the 2008 ITV series in which Buchan starred as a state-sponsored assassin, eliminating bad guys who can't be apprehended by legal means. Your man had a point, I suggest. Buchan disagrees. "I was like, 'What? Does that make us brothers? What I do is illusion. I'm looking at Fernando Torres keyrings and you're just telling me you killed someone.'"

I like the way Buchan dramatises this story, not least because it chimes with another two-hander: the moment when – aged 11 and a pupil at Rivington and Blackrod high school in Lancashire – he realised he wanted to be an actor. "Was it Mrs Pool or Miss Taylor?" he wonders. "Either way, we had a brilliant teacher who made you use your imagination during drama. I played two guys mountain-climbing on the same rope. I knew then I wanted to be an actor."

The Rada graduate, now 35, has realised that dream, having starred in such TV series as Broadchurch, the 18th-century legal drama Garrow's Law, the lobbyist comedy Party Animals, and The Fixer. He is one of Britain's most talented young actors, hailed by critics for his performances on stage and, thanks to his turns on telly, importuned by strangers in the street.

Last year, he says, when the first series of Broadchurch was on ITV, "there was a lot of pinching of my arm and people asking if I was the killer". He played Mark Latimer, father of murdered 11-year-old Danny. How did he reply? "I'd say, 'I'm not going to tell you – but there was a pocket of time where I was unaccounted for."

Was Broadchurch's success (10.47m viewers, lots of Baftas and other awards) a surprise? "It was amazing. You can never really tell if an audience is going to buy it or not."

Here's one reason they did: writer Chris Chibnall broke through mere procedural into what felt shockingly real. In one scene, for instance, Buchan's character takes the most awful walk of his life, heading to the hospital mortuary to identify his son. He's waylayed by a mate outside, collecting signatures for a petition to halt the hospital's closure. Mark signs it and turns down a cheery invitation to play skittles later – obliviousness cheek by jowl with tragedy. Then, unbearably, Mark goes on to ID his boy's corpse.

What made the scene even more striking, I say, was the fact that neither you nor your on-screen wife Beth (played by Jodie Whittaker) have children. "We could only act out what we imagined to be close to the truth," says Buchan, "but, by the same token, you desperately don't want to insult families who have been through that by saying we know what it's like – because we don't."

The second series is filming on Dorset's Jurassic coast. But how could there be a sequel? What is it about? "I can't tell you," he says. "If I did, I would be hunted down. That's true for anybody – David Tennant, Olivia Colman, or any of the new cast." This new cast includes Charlotte Rampling, James D'Arcy and Marianne Jean-Baptiste.

He was perfect as Mark, but it was a struggle landing the part. "Very little gets offered to me," he says. "I have to audition and bawl my eyes out. For Broadchurch, the scene was Danny lying on the mortuary table. I can't remember the last audition I had where I didn't come out drenched in sweat, puffy-eyed."

It was a similar story for the next series in which we will see him, The Honourable Woman, a British spy thriller written and directed by Hugo Blick. "I was cast very late in the day," says Buchan, "and only after a very intense audition." He appears alongside Maggie Gyllenhall, playing Nessa, the daughter of a Zionist arms-dealer who tries to transform her father's firm into a force for peace. She's just been made a baroness – hence the "honourable".

Buchan is Nessa's brother Ephra, a man with a dark secret. "No way I'm telling you what it is," says Buchan. I've watched two episodes and can only think it's something to do with his affair with his child's nanny. "Not even close. I was going to say that bit's the icing on the cake, but it's not even the chocolate sprinkles."

Buchan says Ephra is another character from his roster of men in crisis. "I was Joseph in [BBC drama] The Nativity, a carpenter who gets put through a lot. And in Broadchurch, I'm a plumber who goes through a fair old bit. Same in a film I did with Anna Friel called Having You."

He'll be in crisis yet again in The Great Fire of London, the looming five-parter written by Tom Bradby, political editor of ITV news. Buchan plays Thomas Farriner, the baker whose shop in Pudding Lane started the 1666 conflagration. "It's the most physical thing I've done – a hundredfold more physical than The Fixer. There was no green screen, no CGI, the flames are real. We were running through a proper cauldron. It was absolutely terrifying.

"Oh boo hoo, I say. You're living the dream. "You're right," he laughs. "Initially, it was my dream to get into Rada. Then I got in. So it became my dream to show them I wasn't half bad at acting."

Buchan was born in Stockport in 1979 and grew up in Bolton. His mother died in 2000. His father was head of customs at Manchester airport, tasked with "catching the drug traffickers", as Buchan puts it. "He was always very inspiring to me, still is." It was Dad who encouraged Buchan to do a degree in case acting didn't work out. After graduating from Durham with a 2.1 in modern languages, Buchan returned home.

"I made that classic error of expecting a brass band to be waiting for me, people clapping, when I got back to Bolton. Didn't happen. My first job after graduating was serving chips and beans at Royal Manchester children's hospital. What a wakeup call. I was furious when I was offered that by the agency – how was that using my French? But my dad said, 'Take it' and he was right." He finally got into Rada after a string of other casual jobs: "Burned my forearms cleaning ovens in a Greek restaurant, concierge in a hotel, guide on Granada studios tour … I've done so many jobs before and after uni it's not funny"

In 2012, he married Amy Nuttall, best known for playing man-eater Chloe Atkinson in Emmerdale and housemaid Ethel Parks in Downton Abbey. "I keep that bit private," he says.

He seems fulfilled personally as well as professionally. But some boxes remain unticked. "I want to be in high-stakes intelligent comedy where the man is losing his shit." That's despite his experience with the 2007 series Party Animals, in which he played a Labour party drone turned cynical lobbyist. Though critically acclaimed, it was cancelled after poor ratings.

"I would like to get back to making people laugh. Before drama school, I did nothing but comedy." But then he got drawn, professionally speaking, to the dark side, after seeing Paddy Considine in the 2004 film Dead Man's Shoes. "I phoned my agent and said, 'I'd like to have a go at that kind of thing because there's a definitely a deep well of dark in me.'" He laughs. "Be careful what you wish for."